1. Field of the Invention
The present invention relates to an inexpensive, continuously operating process for the preparation of succinic acid dialkyl esters from maleic acid dialkyl esters in which only very small amounts of the butane-1,4-diol usually formed during hydrogenation of maleic acid esters and no mono- and no hydroxycarboxylic acids of C numbers &lt;4 are formed.
Succinic acid dialkyl esters are important, readily biologically degradable solvents for paints and plasticizers for thermoplastic polyesters having particular mechanical and chemical properties.
2. Description of the Related Art
It is known to prepare succinic acid dialkyl esters by esterification of succinic acid or succinic anhydride with the corresponding monoalcohols, acid components often being employed as esterification catalysts and high excesses of alcohol being used. It is furthermore known to hydrogenate maleic acid dialkyl esters discontinuously with hydrogen in a suspension process with a pulverulent Pd/A.sub.2 O.sub.3 catalyst to give the corresponding succinic acid dialkyl esters (EP 190 424).
The course of the reaction can be illustrated by the following equation: ##STR1##
In this equation, R.sub.1 and R.sub.2 can be identical or different n- or iso-alkyl radicals of C numbers 1 to 12 or cyclic alkyl radicals of C numbers 3 to 6.
Discontinuous processes have the disadvantage that their capacity is very low in relation to the reaction volume and there is therefore a need for large reaction apparatuses and storage tanks. The energy consumption and personnel requirement are relatively high.
Continuous pulverulent catalyst processes which operate with several hydrogenation reactors connected in cascade avoid some of these disadvantages. However, there is still the need for the pulverulent catalysts to be metered in several times in a controlled manner, pumped in circulation and filtered off quantitatively from the reaction product. The catalyst sludge pumps are subject to high mechanical wear. Quantitative removal of the pulverulent catalysts from the reaction product is expensive. Furthermore, the risk of the catalyst activity being reduced relatively rapidly by the additional operations is high. It is therefore advantageous to allow the reaction to proceed over catalysts arranged in a fixed bed. Such catalysts must have a high activity, which should not decrease over a relatively long period of time, because frequent changes of catalyst are likewise expensive in fixed bed reactions.